With recent high growth of industries, membrane technology with a high purity separating function is recognized to be very important, and its importance is increased in various fields including chemical industry, food industry, medical industry, medicine, biochemistry, and environmental fields.
In the field of oil, it is recognized to be important in environmental and economical aspects that various oils, for instance, lubricants, are filtered and recycled during use or after use, and thus are used for a longer period of time.
The illustrative lubricants serve to prevent the irregularities of frictional surfaces from being worn or damaged by high pressure and friction, and to reduce the consumption of power, thus increasing the efficiency of the machine. In addition, lubricating oils have a function to absorb heat generated on frictional surfaces and to discharge the absorbed heat to the outside, thus preventing the frictional surfaces from sticking and becoming damaged. Also, lubricating oils serve to disperse a load concentrated on a contact surface and provide sealing between two frictional surfaces so as to prevent the entry of water or dirt. Moreover, lubricating oils serve to prevent a lubricated surface from being corroded by oxygen and water.
Such lubricating oils are contaminated by worn metal pieces and burned carbides during the operation of machines so that the performance thereof is reduced. For this reason, a unit for filtering and purifying lubricating oil should be provided in systems, or lubricating oil should be replaced with fresh lubricating oil after it has been used for a given period of time. Also, waste lubricating oil may be recycled for reuse as fuel oil or the like.
A method of removing the impurities from the lubricant includes a method of using a centrifugal separator (Korean Patent Application No. 10-2006-7002799, and European Patent Application No. 20040741406), a method of removing fine iron particles using a magnetic force (Korean Patent Application No. 10-1999-0049190), or a method of filtering impurities using an oil filter (Korean Patent Application Nos. 10-1996-040148, 10-2003-0065486, 10-1990-0014047, and 10-2000-7014517, and European Patent Application No. 2008066756). A method of purifying the waste lubricant after use includes a method of using a compound (Korean Patent Application No. 10-1999-0036078), a method of using a catalyst (Korean Patent Application No. 10-2004-0005708), or a method of using a screw (Korean Utility Model Application Nos. 20-2005-0003461, and 20-2005-0003462).
In the purifying methods, however, micro-particles can be removed by separating and filtering, whereas nano-particles cannot be removed. In particular, the nano-particles having a predetermined size reduce the performance of the lubricant, and thus cause a large obstacle when the lubricant is reused. Such nano-particles have a problem in that they reduce not only the performance and life expectancy of the lubricating oil, but also the oil filters. Also, filtration of the lubricating oil needs to be carried out at high temperatures, because lubricating oil has high viscosity. However, conventional oil filters have a problem in that they are deformed at high temperature, because they are based on polymers.